Magnetic exchange coupled MTJ free layer having low switching current and high data retention

ABSTRACT

Embodiments of the invention are directed to a magnetic tunnel junction (MTJ) storage element that includes a reference layer, a tunnel barrier and a free layer on an opposite side of the tunnel barrier layer from the reference layer. The reference layer has a fixed magnetization direction. The free layer includes a first region, a second region and a third region. The third region is formed from a third material that is configured to magnetically couple the first region and the second region. The first region is formed from a first material having a first predetermined magnetic moment, and the second region is formed from a second material having a second predetermined magnetic moment. The first predetermined magnetic moment is lower that the second predetermined magnetic moment.

BACKGROUND

The present invention relates generally to electronic memory, and morespecifically to spin transfer torque (STT) magnetic tunnel junction(MTJ) storage elements having a magnetic exchange coupled composite freelayer configured to minimize the magnitude of a fast switching current(e.g., a write pulse width ≤10 ns) while providing high data retention(e.g., ≥10 years).

Electronic memory can be classified as volatile or non-volatile.Volatile memory retains its stored data only when power is supplied tothe memory, but non-volatile memory retains its stored data withoutconstant power. Volatile random access memory (RAM) provides fastread/write speeds and easy re-write capability. However, when systempower is switched off, any information not copied from volatile RAM to ahard drive is lost. Although non-volatile memory does not requireconstant power to retain its stored data, it in general has lowerread/write speeds and a relatively limited lifetime in comparison tovolatile memory.

Magnetoresistive random access memory (MRAM) is a non-volatile memorythat combines a magnetic device with standard silicon-basedmicroelectronics to achieve the combined attributes of non-volatility,high-speed read/write operations, high read/write endurance and dataretention. The term “magnetoresistance” describes the effect whereby achange to certain magnetic states of the MTJ storage element (or “bit”)results in a change to the MTJ resistance, hence the name“Magnetoresistive” RAM. Data is stored in MRAM as magnetic states orcharacteristics (e.g., magnetization direction, magnetic polarity,magnetic moment, etc.) instead of electric charges. In a typicalconfiguration, each MRAM cell includes a transistor, a MTJ device fordata storage, a bit line and a word line. In general, the MTJ'selectrical resistance will be high or low based on the relative magneticstates of certain MTJ layers. Data is written to the MTJ by applyingcertain magnetic fields or charge currents to switch the magnetic statesof certain MTJ layers. Data is read by detecting the resistance of theMTJ. Using a magnetic state/characteristic for storage has two mainbenefits. First, unlike electric charge, magnetic state does not leakaway with time, so the stored data remains even when system power isturned off. Second, switching magnetic states has no known wear-outmechanism.

STT is a phenomenon that can be leveraged in MTJ-based storage elementsto assist in switching the storage element from one storage state (e.g.,“0” or “1”) to another storage state (e.g., “1” or “0”). For example,STT-MRAM 100 shown in FIG. 1 uses electrons that have beenspin-polarized to switch the magnetic state (i.e., the magnetizationdirection 110) of a free layer 108 of MTJ 102. The MTJ 102 is configuredto include a reference/fixed magnetic layer 104, a thin dielectrictunnel barrier 106 and a free magnetic layer 108. The MTJ 102 has a lowresistance when the magnetization direction 110 of its free layer 108 isparallel to the magnetization direction 112 of its fixed layer 104.Conversely, the MTJ 102 has a high resistance when its free layer 108has a magnetization direction 110 that is oriented anti-parallel to themagnetization direction 112 of its fixed layer 104. STT-MRAM 100includes the multi-layered MTJ 102 in series with the FET 120, which isgated by a word line (WL) 124. The BL 126 and a source line (SL) 128can, depending on the design, run parallel to each other. The BL 126 iscoupled to the MTJ 102, and the SL 128 is coupled to the FET 120. TheMTJ 102 (which is one of multiple MTJ storage elements along the BL 126)is selected by turning on its WL 124.

The MTJ 102 can be read by activating its associated word linetransistor (e.g., field effect transistor (FET) 120), which switchescurrent from a bit line (BL) 126 through the MTJ 102. The MTJ resistancecan be determined from the sensed current, which is itself based on thepolarity of the magnetization direction 110 of the free layer 108.Conventionally, if the magnetization directions 112, 110 of the fixedlayer 104 and the free layer 108 have the same polarities, theresistance is low and a “0” is read. If the magnetization directions112, 110 of the fixed layer 104 and the free layer 108 have oppositepolarities, the resistance is higher and a “1” is read.

When a voltage (e.g., 500 mV) is forced across the MTJ 102 from the BL126 to the SL 128, current flows through the selected cell's MTJ 102 towrite it into a particular state, which is determined by the polarity ofthe applied voltage (BL high vs. SL high). During the write operation,spin-polarized electrons generated in the reference layer 104 tunnelthrough the tunnel layer 106 and exert a torque on the free layer 108,which can switch the magnetization direction 110 of the free layer 108.Thus, the amount of current required to write to a STT-MRAM MTJ dependson how efficiently spin polarization is generated in the MTJ.Additionally, STT-MRAM designs that keep write currents small (e.g.,I_(c)<25 micro-ampere) are important to improving STT-MRAM scalability.This is because a larger switching current would require a largertransistor (e.g., FET 120), which would inhibit the ability to scale upSTT-MRAM density.

However, in order to achieve fast switching (e.g., a write pulse width≤10 ns) in STT MRAM devices, a large current is needed. Morespecifically, in a fast switching regime, a so-called overdrive current,which is the difference between the switching current I_(c) at a certainpulse width and the critical current I_(c0), is inversely proportionallyto the write pulse, as shown in Equation (1).

$\begin{matrix}{{\eta\frac{I_{c} - I_{c\; 0}}{e}t_{p}} \propto \frac{m}{\mu_{B}}} & {{Equation}\mspace{14mu}(1)}\end{matrix}$

In Equation (1), I_(c)-I_(c0) is the overdrive current, η is the spinpolarization of the magnetic materials, t_(p) is the pulse width, m isthe total moment of the free layer material, and μ_(B) is the Bohrmagneton, which is a constant. Equation (1) suggests that to minimizethe switching current at a certain pulse width, it is necessary toreduce the free layer moment (m). Providing a low moment free layer isalso advantageous for improving the MTJ's deep bit write error rate(WER) performance.

However, simple low moment free layers suffer from low activation energy(E_(b)), which results in poor retention. In general, the activationenergy is the amount of energy required to flip the MTJ free layer'smagnetic state. In order to retain data that has been written to the MTJfree layer, the activation energy must be sufficiently high to preventrandom energy sources (e.g., heat) in the MTJ's operating environmentfrom unintentionally applying enough energy to flip the MTJ free layer.Accordingly, in known STT-MRAM operating in a fast switching regime,minimizing the switching current through a low moment free layer has theundesirable result of lowering activation energy (E_(b)) and dataretention (e.g., ≤10 years).

SUMMARY

Embodiments of the invention are directed to a magnetic tunnel junction(MTJ) storage element. In a non-limiting example, the MTJ includes areference layer, a tunnel barrier and a free layer on an opposite sideof the tunnel barrier layer from the reference layer. The referencelayer has a fixed magnetization direction. The free layer includes afirst region, a second region and a third region. The third region isformed from a third material that is configured to magnetically couplethe first region and the second region. The first region is formed froma first material having a first predetermined magnetic moment, and thesecond region is formed from a second material having a secondpredetermined magnetic moment. The first predetermined magnetic momentis lower that the second predetermined magnetic moment. Advantages ofthe above-described embodiments of the invention include, but are notlimited to, providing the ability to use the first region to switch thesecond region using magnetic exchange coupling provided by the thirdregion.

Embodiments of the invention are directed to a MTJ storage element. In anon-limiting example, the MTJ includes a reference layer, a tunnelbarrier and a free layer on an opposite side of the tunnel barrier layerfrom the reference layer. The reference layer has a fixed magnetizationdirection. The free layer includes a first region and a second regionseparated by a third region. The third region is formed from a thirdmaterial that is configured to magnetically couple the first region andthe second region. The first region is formed from a first materialhaving a first predetermined magnetic moment and a first predeterminedactivation energy. The second region is formed from a second materialhaving a second predetermined magnetic moment and a second predeterminedactivation energy. The first predetermined magnetic moment is lower thatthe second predetermined magnetic moment, and the second predeterminedactivation energy is higher than the first predetermined activationenergy. Advantages of the above-described embodiments of the inventioninclude, but are not limited to, providing the ability to use the firstregion to switch the second region using magnetic exchange couplingprovided by the third region, as well as providing a free layer having alow moment region and a high activation energy region. The low momentallows the switching current of the free layer to remain low, and thehigh activation region provides a higher data retention of the freelayer.

Embodiments of the invention are directed to a MTJ storage element. In anon-limiting example, the MTJ includes a reference layer, a tunnelbarrier and a free layer on an opposite side of the tunnel barrier layerfrom the reference layer. The reference layer has a fixed magnetizationdirection. The free layer includes a first region and a second regionseparated by a spacer region. The spacer region is configured to providea predetermined magnetic exchange coupling strength between the firstregion and the second region. The first region is formed from a firstmaterial having a first predetermined magnetic moment and a firstpredetermined activation energy. The second region is formed from asecond material having a second predetermined magnetic moment and asecond predetermined activation energy. The first predetermined magneticmoment is configured to be lower than the second predetermined magneticmoment, and the second predetermined activation energy is configured tobe higher than the first predetermined activation energy. Advantages ofthe above-described embodiments of the invention include, but are notlimited to, providing the ability to use the first region to switch thesecond region using magnetic exchange coupling provided by the spacerregion, as well as providing a free layer having a low moment region anda high activation energy region. The low moment allows the switchingcurrent of the free layer to remain low, and the high activation regionprovides a higher data retention of the free layer.

Embodiments are directed to a method of forming a MTJ storage element. Anon-limiting example method includes forming a reference layer having afixed magnetization direction, a tunnel barrier layer, and a compositefree layer on an opposite side of the tunnel barrier layer from thereference layer. Forming the free layer includes forming a first regionand a second region separated by a spacer region. Forming the firstregion further includes configuring the first region to include a firstpredetermined magnetic moment and a first predetermined activationenergy. Forming the second region further includes configuring thesecond region to include a second predetermined magnetic moment and asecond predetermined activation energy. The first predetermined magneticmoment is configured to be lower than the second predetermined magneticmoment, and the second predetermined activation energy is configured tobe higher than the first predetermined activation energy. Forming thethird region further includes configuring the third region tomagnetically couple the first region and the second region. Advantagesof the above-described embodiments of the invention include, but are notlimited to, providing the ability to use the first region to switch thesecond region using magnetic exchange coupling provided by the thirdregion.

Embodiments are directed to methods of operating a MTJ storage element.A non-limiting example method includes applying a write pulse having apredetermined magnitude to an MTJ storage element that includes areference layer having a fixed magnetization direction, a tunnel barrierlayer, and a free layer on an opposite side of the tunnel barrier layerfrom the reference layer. The free layer includes a first region, asecond region, and a spacer material between the first region and thesecond region. The first region includes a first material configured toinclude a first switchable magnetization direction. The second regionincludes a second material configured to include a second switchablemagnetization direction. The spacer material is configured to provideexchange magnetic coupling between the first region and the secondregion. The method further includes, based at least in part on receivingthe write pulse, generating in the reference layer an amount of spintorque electrons that is insufficient to initiate a process of switchingthe switchable magnetization direction of the second region. The methodfurther includes, based at least in part on the spin torque electronsgenerated in the reference layer material, initiating a process ofswitching the switchable magnetization direction of the first region.The method further includes, based at least in part on the switchablemagnetization direction of the first region switching, initiating aprocess of switching the switchable magnetization direction of thesecond region based at least in part on the spacer material providingmagnetic exchange coupling between the first region and the secondregion. Advantages of the above-described embodiments of the inventioninclude, but are not limited to, providing the ability to use the firstregion to switch the second region using magnetic exchange couplingprovided by the third region.

Additional features and advantages are realized through the techniquesof the present invention. Other embodiments and aspects of the inventionare described in detail herein and are considered a part of the claimedsubject matter. For a better understanding, refer to the description andto the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The diagrams depicted herein are illustrative. There can be manyvariations to the diagram or the operations described therein withoutdeparting from the spirit of the invention. For instance, the actionscan be performed in a differing order or actions can be added, deletedor modified. Also, the term “coupled” describes having a signal pathbetween two elements and does not imply a direct connection between theelements with no intervening elements/connections therebetween. All ofthese variations are considered a part of the specification.

The subject matter of the invention is particularly pointed out anddistinctly claimed in the claims at the conclusion of the specification.The foregoing and other features and advantages are apparent from thefollowing detailed description taken in conjunction with theaccompanying drawings in which:

FIG. 1 depicts a block diagram of a STT-MRAM capable of utilizing amagnetic exchange coupled spin torque MTJ storage element configuredaccording to embodiments of the present invention;

FIG. 2 depicts a block diagram of a magnetic exchange coupled spintorque MTJ configured according to embodiments of the present invention;

FIG. 3 depicts a block diagram of a magnetic exchange coupled spintorque MTJ configured according to embodiments of the present invention;

FIG. 4A depicts a write pulse that can be applied to a magnetic exchangecoupled spin torque MTJ configured according to embodiments of theinvention;

FIG. 4B depicts a sequence of diagrams illustrating a non-limitingexample of a write operation of a magnetic exchange coupled spin torqueMTJ configured according to embodiments of the invention; and

FIG. 5 depicts a flow diagram illustrating a method of forming amagnetic exchange coupled spin torque MTJ according to embodiments ofthe invention.

DETAILED DESCRIPTION

Various embodiments of the present invention are described herein withreference to the related drawings. Alternative embodiments of theinvention can be devised without departing from the scope of thisinvention. It is noted that various connections and positionalrelationships (e.g., over, below, adjacent, etc.) are set forth betweenelements in the following description and in the drawings. Theseconnections and/or positional relationships, unless specified otherwise,can be direct or indirect, and the present invention is not intended tobe limiting in this respect. Accordingly, a coupling of entities canrefer to either a direct or an indirect coupling, and a positionalrelationship between entities can be a direct or indirect positionalrelationship. As an example of an indirect positional relationship,references in the present description to forming layer “A” over layer“B” include situations in which one or more intermediate layers (e.g.,layer “C”) is between layer “A” and layer “B” as long as the relevantcharacteristics and functionalities of layer “A” and layer “B” are notsubstantially changed by the intermediate layer(s).

For the sake of brevity, conventional techniques related to MTJfabrication may or may not be described in detail herein. Moreover, thevarious tasks and process steps described herein can be incorporatedinto a more comprehensive procedure or process having additional stepsor functionality not described in detail herein. In particular, varioussteps in the manufacture of STT-MRAM and MTJ devices are well known andso, in the interest of brevity, many conventional steps are onlymentioned briefly herein or are omitted entirely without providing thewell-known process details.

STT-MRAM, which utilizes the spin transfer torque (STT) effect to switchits MTJ free layer magnetic state, combines high speed, high density,nonvolatility, scalability, and endurance. The MTJ free layer, which isthe primary memory element, is typically formed from MgO-based materialsdue to their fairly low switching-current density (compared withmetallic spin valves) as well as large resistance and tunnelingmagnetoresistance (TMR) ratio compatible with read and write operationsin integrated CMOS technology. For optimal operation, STT-MRAM preventsfalse-switching events (e.g., unintended thermal activation) whileminimizing the energy dissipation during current-induced switching ofthe MTJ free layer (i.e., the write energy).

Minimizing the write energy while also providing an activation energy orenergy barrier (E_(b)) that is sufficiently high to preventfalse-switching events is a challenge in high-speed memory applicationshaving write times in the nanosecond range (e.g., a write pulse width≤10 ns). In high-speed memory applications, the required switchingcurrent densities are high compared with those in quasi-static orlong-pulse switching. As previously described herein, in a fastswitching regime, a so-called overdrive current, which is the differencebetween the switching current I_(c) at a certain pulse width and thecritical current I_(c0)), is inversely proportionally to the writepulse, as shown in Equation (1).

$\begin{matrix}{{\eta\frac{I_{c} - I_{c\; 0}}{e}t_{p}} \propto \frac{m}{\mu_{B}}} & {{Equation}\mspace{14mu}(1)}\end{matrix}$

In Equation (1), I_(c)-I_(c0) is the overdrive current, η is the spinpolarization of the magnetic materials, t_(p) is the pulse width, m isthe total moment of the free layer material, and μ_(B) is the Bohrmagneton, which is a constant. Equation (1) suggests that to minimizethe switching current at a certain pulse width, it is necessary toreduce the free layer moment (m). Providing a low moment free layer isalso advantageous for improving the MTJ's deep bit write error rate(WER) performance.

However, simple low moment free layers suffer from low activation energy(E_(b)), which results in poor retention. In general, the activationenergy is the amount of energy required to flip the MTJ free layer'smagnetic state. In order to retain data that has been written to the MTJfree layer, the activation energy must be sufficiently high to preventrandom energy sources (e.g., heat) in the MTJ's operating environmentfrom unintentionally applying enough energy to flip the MTJ free layer.Accordingly, in known STT-MRAM operating in a fast switching regime,minimizing the switching current through low moment free layers has theundesirable result of lowering activation energy (E_(b)) and dataretention (e.g., ≤10 years).

Turning now to an overview of aspects of the invention, embodiments ofthe invention are directed to STT-MRAM that provides MTJ storageelements having a magnetic exchange coupled composite free layerconfigured to minimize the required magnitude of a fast switchingcurrent (e.g., a write pulse width ≤10 ns) while providing high dataretention (e.g., ≥10 years). In some embodiments of the invention, thefree layer is formed from a composite structure having a low momentregion and a high energy barrier (E_(b)) region. In some embodiments,magnetic exchange coupling is provided between the low moment region andthe high E_(b) region by a non magnetic spacer layer positioned betweenthe low moment region and the high E_(b) region. In some embodiments,the low moment region is adjacent to the MTJ tunnel barrier, and the MTJtunnel barrier is adjacent to the MTJ fixed layer.

The magnitude of the write pulse is selected to be sufficient to beginand complete the switching of the low moment region's magnetizationdirection. However, the magnitude of the write pulse is also selected tobe insufficient to switch the high E_(b) region's magnetizationdirection. Accordingly, the magnitude of the write pulse only needs tobe high enough to generate enough spin torque electrons in thereference/fixed layer to tunnel through the tunnel barrier and into thelow moment region to begin and complete the switching of the low momentregion's magnetization direction. As previously described herein,Equation (1) suggests that, at a fast-switching pulse width, the factthat the low moment region is formed from a low moment materialminimizes the required switching current.

Because the low moment region is magnetically coupled to the high E_(b)region, as the low moment region's magnetization direction is switchedit drags the high E_(b) region's magnetization direction to switch alongwith it. Accordingly, the low moment region's magnetization direction isswitched by the write current, and the high E_(b) region's magnetizationdirection is switched by the low moment region through the effects ofmagnetic exchange coupling. The novel magnetic exchange coupled freelayer has both low overdrive, which is determined by the low momentregion at the tunnel barrier interface, and high E_(b), which largelycomes from the high E_(b) region. By tuning the parameters (e.g.,thickness, material, etc.) of the nonmagnetic spacer layer to optimizethe magnetic exchange coupling strength, and by positioning thenonmagnetic spacer layer between the low moment region and the highE_(b) region of the free layer, an optimum switching efficiency(E_(b)/I_(c) at 10 ns) can be achieved.

The low moment region can be Co, Fe, Ni, and B based materials with orwithout light element doping. As used herein, the phrase “light elementdoping” refers to a doping level of about 10% of the host material. Insome embodiments of the invention, light element doping is in the rangefrom about 5% to about 30% of the host material. Examples dopantsinclude, but are not limited to, Al, Mg, Ti, Sc, Ca, V, Cr, Mn, Ge, Si,C, Be and Ga. The low moment region's thickness can range from about 8angstroms to about 20 angstroms. The high E_(b) region can be Co, Fe, Niand B based materials; Co/Pd, Co/Ir, Co/Pt, Co(Fe)/Tb, Co(Fe)/Gd, Co/Rhmultilayers; or CoPt, CoPd, FePt, FePd, CoFeTb, CoFeGd alloys. The highE_(b) region's thickness can range from about 15 angstroms to about 100angstroms. The nonmagnetic spacer can be a metal, including, forexample, Ta, W, Jr, Mo and its alloys with Fe, Co or Ni. The nonmagneticspacer can also be an oxide, including, for example, MgO, AlOx, TiOx,TaOx, WOx or mixtures thereof. The nonmagnetic spacer layer's thicknesscan range from about 2 angstroms to about 20 angstroms.

The thicknesses of the respective layers of the novel MTJ storageelements described herein can vary according to design considerations.For example, the thicknesses of the layers of the novel MTJ storageelement can be designed to have predetermined thicknesses, to havethicknesses within predetermined ranges, to have thicknesses havingfixed ratios with respect to each other, or to have thicknesses based onany other consideration or combination of considerations in accordancewith the various functionalities described herein.

The magnetic exchange coupled free layer can be over the tunnel barrieror underneath the tunnel barrier. In either configuration, the lowmoment region is adjacent to the tunnel barrier in order to minimize theoverdrive current.

Turning now to a more detailed description of aspects of the presentinvention, FIG. 2 depicts a block diagram illustrating an exampleconfiguration of a magnetic exchange coupled spin torque MTJ-basedstorage element 102A according to embodiments of the present invention.The MTJ 102A can be implemented in the STT-MRAM 100 (shown in FIG. 1) inthe same manner as the MTJ 102. The MTJ 102A includes a magneticreference layer 204, a dielectric tunnel barrier 206, and a compositefree layer 208, configured and arranged as shown. The composite freelayer 208 includes a first region 210, a second region 212, and a thirdregion 214 disposed between the first region 210 and the second region212. In some embodiments of the invention, the first region 210 isimplemented as a low-moment magnetic layer, the second region 212 isimplemented as a high E_(b) magnetic layer, and the third region 214 isimplemented as a nonmagnetic spacer layer. The magnetic layers that formthe MTJ 102A have perpendicular magnetization, which means that themagnetization directions of all of the magnetic layers are perpendicularto the plane of the film (either up or down). The free layers 210, 212have their magnetization directions parallel to each other (when notbeing written), i.e., all are up or all are down.

The magnetic reference layer 204 is formed and configured such that itsmagnetization direction 220 is fixed. The first and second regions areformed and configured in a manner that provides them with switchablemagnetization directions 222, 224. The third region 214 is nonmagnetic.However, the third region 214 is configured to magnetically couple thefirst region 210 and the second region 212. More specifically, the thirdregion is configured to provide a predetermined magnetic exchangecoupling strength from the first region 210 across the third region 214to the second region 212. The parameters (e.g., material, thickness,etc.) of the third region 214 are selected such that the resultingpredetermined magnetic exchange coupling results in optimal switchingefficiency (E_(b)/I_(c) at 10 ns). In some embodiments, the third region214 can be a nonmagnetic spacer formed from metal, including, forexample, Ta, W, Jr, Mo and its alloys with Fe, Co or Ni. The thirdregion 214 can also be an oxide, including, for example, MgO, AlOx,TiOx, TaOx, WOx or mixtures thereof. The thickness of the third region214 can range from about 2 angstroms to about 20 angstroms.

The first region 210 can be formed from a low moment magnetic material.A magnetic material can be considered “low moment” if its MA product isabout 0.12 emu/cm², where M_(s) is the material's saturationmagnetization, t is the material's thickness, and emu is a cgs(centimetre-gram-second) unit for measuring magnetic moment. The firstregion 210 can be Co, Fe, Ni, and B based materials with or withoutlight element doping. As used herein, the phrase “light element doping”refers to a doping level of about 10% of the host material. In someembodiments of the invention, light element doping is in the range fromabout 5% to about 30% of the host material. Examples dopants include,but are not limited to, Al, Mg, Ti, Sc, Ca, V, Cr, Mn, Ge, Si, C, Be andGa. The thickness of the first region 210 can range from about 8angstroms to about 20 angstroms. In general, a layer of material'smagnetic moment is related to the layer's thickness such that thethinner the layer the lower the magnetic moment. Accordingly, thethickness of the first region 210 is selected to achieve a desiredmagnetic moment, which is selected to achieve a desired switchingcurrent for the first region 210.

The second region 212 can be formed from a high E_(b) magnetic layer. Amagnetic layer can be considered to have a high E_(b) if its E_(b) issufficient to satisfy the data retention requirement for targetedapplication. This translates to an activation energy (or energy barrier)E_(b) that is within the range from about 60 to about 100 kT (k=theBoltzmann constant, and T=temperature) for approximately 10 year dataretention. The second region 212 can be Co, Fe, Ni and B basedmaterials; Co/Pd, Co/Ir, Co/Pt, Co(Fe)/Tb, Co(Fe)/Gd, Co/Rh multilayers;or CoPt, CoPd, FePt, FePd, CoFeTb, CoFeGd alloys. The thickness of thesecond region 212 can range from about 15 angstroms to about 100angstroms. In general, a layer of material's energy barrier E_(b) isrelated to the layer's thickness such that the thicker the layer thehigher the energy barrier E_(b). Accordingly, the thickness of thesecond region 212 is selected to achieve a desired energy barrier E_(b),which is selected to meet a desired data retention requirement for thetarget application.

The tunnel barrier layer 206 can be formed from a relatively thin (e.g.,about 10 angstroms) layer of dielectric material (e.g., MgO). When twoconducting electrodes (e.g., reference/fixed layer 204 and the firstregion 210) are separated by a thin dielectric layer (e.g., tunnelbarrier layer 206), electrons can tunnel through the dielectric layerresulting in electrical conduction. The electron tunneling phenomenonarises from the wave nature of the electrons, and the resulting junctionelectrical conductance is determined by the evanescent state of theelectron wave function within the tunnel barrier. Accordingly, thetunnel barrier 206 is configured to be thin enough to allow electrons(specifically, spin torque electrons) from the reference layer 204 toquantum mechanically tunnel through the tunnel barrier 206. The tunnelbarrier 206 is also configured to be thick enough to decouple the firstregion 210 of the composite free layer 208 from the reference layer 204such that the magnetization direction 222 of the first region 210 isfree to flip back and forth.

With the optimal exchange coupling strength and composite free layerdesign, the magnitude of the write pulse needed to switch the wholeexchange coupled composite free layer 208 will be lower than what isneeded to switch a simple free layer with the same total activationenergy E_(b). Thus the exchange coupled composite free layer 208improves switching efficiency defined as E_(b)/I_(c). The composite freelayer structure also improves the write-error rate performance of theMTJ 102A. A write pulse applied to the MTJ 102A, according toembodiments of the invention, needs to be sufficient to begin switchingthe low moment first region's magnetization direction 222 butinsufficient to switch the high E_(b) second region's magnetizationdirection 224.

Because the low moment first region 210 is magnetically coupled to thehigh E_(b) second region 212, as the low moment region's magnetizationdirection 222 is switched it drags the high E_(b) region's magnetizationdirection 224 to switch along with it. Accordingly, the low momentregion's magnetization direction 222 is switched by the write current,and the high E_(b) region's magnetization direction 224 is switched bythe low moment first region 210 through the effects of magnetic exchangecoupling. The novel magnetic exchange coupled composite free layer 208,according to embodiments of the invention, has both low overdrive, whichis determined by the low moment region 210 at the tunnel barrierinterface, and high E_(b), which largely comes from the high E_(b)region 212. By optimizing the magnetic exchange coupling strengthbetween the low moment first region 210 and the high E_(b) second region212 of the composite free layer 208 across the nonmagnetic third region214, an optimum switching efficiency (E_(b)/I_(c) at 10 ns) can beachieved.

FIG. 3 depicts a block diagram illustrating an example configuration ofa magnetic exchange coupled spin torque MTJ-based storage element 102Baccording to embodiments of the present invention. The MTJ 102B can beimplemented in the STT-MRAM 100 (shown in FIG. 1) in the same manner asthe MTJ 102. The MTJ 102B is substantially the same as the MTJ 102A(shown in FIG. 2) except the composite free layer 208 of the MTJ 102B isunderneath the tunnel barrier 206 in the MTJ 102B. The MTJ 102B operatesin substantially the same manner as the MTJ 102A because in either MTJ102A or MTJ 102B the low moment first region 210 is adjacent to thetunnel barrier 206 in order to minimize the overdrive current requiredto begin switching the first region 210.

A non-limiting example of the write operation of the MTJ 102B will nowbe described with reference to FIGS. 4A and 4B. Although described inconnection with MTJ 102B, the write operations apply equally to the MTJ102A with appropriate modifications to take into account that thecomposite free layer 208 of the MTJ 102A is over the tunnel barrier 206in the MTJ 102A. FIG. 4A depicts a diagram of a write pulse 450according to embodiments of the invention. The write pulse 450 isapplied to the MTJ 104B (shown in FIGS. 3 and 4B) and operates,according to embodiments of the invention, to change the magnetizationdirection 222 of the low moment first region 210 of the free layer 208from pointing up to pointing down (or from pointing down to pointingup). However, the write pulse magnitude of the write pulse 450 isselected, according to embodiments of the invention, to also beinsufficient to change the magnetization direction 224 of the high E_(b)second region 212 of the free layer 208. For comparison, a write pulse452 is depicted to show an example of the higher write pulse magnitudethat would be required if write current were used to switch a high E_(b)free layer. The write pulses 450, 452 are depicted in a diagram/graphthat shows the pulse magnitudes of the write pulses 450, 452 over time(t). In the depicted embodiment, the write pulses 450, 452 include awrite pulse duration of about ten (10) nanoseconds, which places thewrite pulses 450, 452 within what is generally considered a“fast-switching” regime.

FIG. 4B depicts a sequence of diagrams illustrating a non-limitingexample of how the magnetization directions 220, 222, 224 of the MTJ102B can change over time during the initial application of a writepulse (i.e., switching current) such as write pulse 450 according toembodiments of the invention. FIG. 4B depicts a sequence of fourdiagrams along the top of FIG. 4B, where each diagram illustrates themagnetization directions 220, 222, 224 of the MTJ 102B at a particulartime (t) and with a downward current direction (e−) during applicationof the write pulse 450 shown in FIG. 4A. Examples of the magnetizationdirections 220, 222, 224 of the MTJ 102B are depicted at t=zero (0) ns,t=2.0 ns, t=5.5 ns, and t=10 ns. The diagrams shown for each time arefor illustration purposes and are not intended to convey precisepositions of the magnetization directions 220, 222, 224 at the exactpoints in time of zero (0) ns, 2.0 ns, 5.0 ns, and 10.0 ns. Instead, thediagrams shown in FIG. 4B are intended to convey an example of how thegeneral and relative progression of the changes in the magnetizationdirections 220, 222, 224 can occur according to embodiments of theinvention, and are not intended to convey the specific times at whichthe changes occur, or the specific order of the changes.

The diagram at t=zero (0) depicts the magnetization directions 220, 222,224 of the MTJ 102B at the start time of the write pulse 450. At t=2.0ns the write pulse 450 has started the process of switching themagnetization direction 222 of the low moment first region 210. However,the magnitude of the write pulse 450 is insufficient to switch the highE_(b) second region 212. At t=5.0 ns, the write pulse 450 continues theprocess of changing the magnetization direction 222 of the low momentfirst region 210. While the magnitude of the write pulse 450 isinsufficient to switch the high E_(b) second region 212, by t=5.0 ns,the, the magnetization direction 222 of the low moment first region 210has begun to influence the magnetization direction 224 of the high E_(b)second region 212 through magnetic exchange coupling provided by thenonmagnetic third region 214. Because the low moment first region 210 ismagnetically coupled to the high E_(b) second region 212, as the lowmoment region's magnetization direction 222 is switched it drags thehigh E_(b) region's magnetization direction 224 to switch along with it.Accordingly, the low moment region's magnetization direction 222 isswitched by the write current 450, and the high E_(b) region'smagnetization direction 224 is switched by the low moment region 210through the effects of magnetic exchange coupling exerted by thenonmagnetic third region 214. The novel magnetic exchange coupled freelayer 208 has both low overdrive, which is determined by the low momentfirst region 210 at the interface with the tunnel barrier 206, and highE_(b), which largely comes from the high E_(b) second region 212. Bytuning the parameters (e.g., thickness, material, etc.) of thenonmagnetic third region 214 to optimize the magnetic exchange couplingstrength and by positioning the nonmagnetic third region 214 between thelow moment first region 210 and the high E_(b) second region 212 of thecomposite free layer 208, an optimum switching efficiency (E_(b)/I_(c)at 10 ns) can be achieved. By t=10.0 ns, the novel process of switchingthe composite free layer's magnetization directions 222, 224 hascompleted.

FIG. 4B also depicts a sequence of four diagrams along the bottom ofFIG. 4B, where each diagram illustrates the magnetization directions220, 222, 224 of the MTJ 102B at a particular time (t) and with anupward current direction (e−) during application of a negative versionof the write pulse 450 shown in FIG. 4A. The switching operationdepicted by the sequence of four diagrams along the bottom of FIG. 4Bproceeds in substantially the same manner as the switching operationdepicted by the sequence of four diagrams along the top of FIG. 4B,except the current direction is upward, and the composite free layermagnetization directions 222, 224 are switched from pointing downward topointing upward.

FIG. 5 depicts a flow diagram illustrating a method 500 of forming theMTJ 102A according to embodiments of the invention. In block 502, thereference layer 204 is formed. The reference layer 204 can be formed,for example, by any suitable deposition, growth or other formationprocess. The reference layer 204 can be formed of ferromagneticmaterial, including, but not limited to, Co containing multilayers forexample Co|Pt, Co|Ni, Co|Pd, Co|Ir, Co|Rh etc, alloys with perpendicularanisotropy for example CoPt, FePt, CoCrPt etc and rare earth-transitionmetals, for example CoFeTb etc. In accordance with embodiments of theinvention, the reference layer 212 is formed and configured in a mannerthat provides the fixed magnetization direction 220.

In block 504, the tunnel barrier layer 206 is formed over the referencelayer 204. The tunnel barrier layer 206 can be formed, for example, byany suitable deposition, growth or other formation process, and can be anon-conductive material, including, but not limited to, MgO, AlOx,MgAlOx, CaOx etc. In accordance with embodiments of the invention, thetunnel barrier 206 is configured to be thin enough to allow electrons(specifically, spin torque electrons) from the reference layer 204 toquantum mechanically tunnel through the tunnel barrier 206. The tunnelbarrier 206 is also configured to be thick enough to decouple thecomposite free layer 208 from the reference layer 204 such that themagnetization directions 222, 224 of the composite free layer 208 arefree to flip back and forth.

In block 506, the composite free layer 208 is formed over the tunnelbarrier layer 206. The composite free layer 208, in accordance withembodiments of the present invention, includes a low moment first region210, a high E_(b) second region 212 and a nonmagnetic third region 214positioned between the first region 210 and the second region 212. Thecomposite free layer 208 can be formed by forming the low moment firstregion 210 over the tunnel barrier 206, forming the nonmagnetic thirdregion 214 over the first region 210, and forming the high E_(b) thirdregion 212 over the third region 214.

The first, second and third regions, 210, 212, 214 can be formed, forexample, by any suitable deposition, growth or other formation process.The low moment first region 210 can be Co, Fe, Ni, and B based materialswith or without light element doping. As used herein, the phrase “lightelement doping” refers to a doping level of about 10% of the hostmaterial. In some embodiments of the invention, light element doping isin the range from about 5% to about 30% of the host material. Examplesdopants include, but are not limited to, Al, Mg, Ti, Sc, Ca, V, Cr, Mn,Ge, Si, C, Be and Ga. The thickness of the low moment first region 210can range from about 8 angstroms to about 20 angstroms. The high E_(b)second region 212 can be Co, Fe, Ni and B based materials; Co/Pd, Co/Ir,Co/Pt, Co(Fe)/Tb, Co(Fe)/Gd, Co/Rh multilayers; or CoPt, CoPd, FePt,FePd, CoFeTb, CoFeGd alloys. The thickness of the high E_(b) secondregion 212 can range from about 15 angstroms to about 100 angstroms. Thenonmagnetic third region 214 can be a metal, including, for example, Ta,W, Jr, Mo and its alloys with Fe, Co or Ni. The nonmagnetic third region214 can also be an oxide, including, for example, MgO, AlOx, TiOx, TaOx,WOx or mixtures thereof. The thickness of the nonmagnetic third region214 can range from about 2 angstroms to about 20 angstroms. Inaccordance with embodiments of the invention, the first region 210 andthe second region 212 of the composite free layer 208 are formed andconfigured in a manner that provides the switchable magnetizationdirections 222, 224.

The first region 210 can be formed from a low moment magnetic metal. Amagnetic metal can be considered “low moment” if its M_(s)t product is≤about 0.12 emu/cm², where M_(s) is the material's saturationmagnetization, t is the material's thickness, and emu is a cgs(centimetre-gram-second) unit for measuring magnetic moment.

The second region 212 can be formed from a high E_(b) magnetic metal. Amagnetic metal can be considered to have a high E_(b) if its E_(b) issufficient to satisfy the data retention requirement for targetedapplication. This translates to an activation energy (or energy barrier)E_(b) that is within the range from about 60 to about 100 kT (k=theBoltzmann constant, and T=temperature) for approximately 10 year dataretention.

The thicknesses of the respective layers of the novel MTJ storageelements described herein can vary according to design considerations.For example, the thicknesses of the layers of the novel MTJ storageelement can be designed to have predetermined thicknesses, to havethicknesses within predetermined ranges, to have thicknesses havingfixed ratios with respect to each other, or to have thicknesses based onany other consideration or combination of considerations in accordancewith the various functionalities described herein.

The magnetic exchange coupled free layer can be over the tunnel barrieror underneath the tunnel barrier. In either configuration, the lowmoment region can be adjacent to the tunnel barrier in order to minimizethe overdrive current.

Thus it can be seen from the foregoing detailed description that thepresent invention provides STT-MTJ storage elements having a magneticexchange coupled composite free layer configured to minimize themagnitude of a fast switching current (e.g., a write pulse width ≤10 ns)while providing high data retention (e.g., ≥10 years). The compositefree layer includes a low moment first region, a high E_(b) secondregion, and a nonmagnetic third region separating the first and secondregions. The magnitude of the fast switching write pulse is selected tobe sufficient to begin the switching of the low moment region'smagnetization direction. By optimizing the exchange coupling strength ofthe third region, as well as parameters (region thicknesses, magneticmoment, activation energy, etc.) of the overall composite free layerdesign, the magnitude of the write pulse needed to switch the wholeexchange coupled composite free layer will be lower than what is neededto switch a simple free layer with the same total activation energyE_(b). Thus, the exchange coupled composite free layer improvesswitching efficiency defined as E_(b)/I_(c) at 10 ns. The composite freelayer structure also improves device write-error rate performance.

Because the low moment first region is magnetically coupled to the highE_(b) second region, as the low moment first region's magnetizationdirection is switched it drags the high E_(b) second region'smagnetization direction to switch along with it. Accordingly, the lowmoment first region's magnetization direction is switched by the writecurrent, and the high E_(b) second region's magnetization direction isswitched by the low moment first region through the effects of magneticexchange coupling. The novel magnetic exchange coupled composite freelayer has both low overdrive, which is determined by the low momentfirst region at the tunnel barrier interface, and high E_(b), whichlargely comes from the high E_(b) second region. By tuning theparameters (e.g., width, material, etc.) of the nonmagnetic third regionto optimize the magnetic exchange coupling strength and optionally thecurrent isolation properties of the nonmagnetic spacer layer, and bypositioning the nonmagnetic third region between the low moment firstregion and the high E_(b) second region of the composite free layer, anoptimum switching efficiency (E_(b)/I_(c) at 10 ns) can be achieved.

The terms “example” or “exemplary” are used herein to mean “serving asan example, instance or illustration.” Any embodiment or designdescribed herein as “exemplary” is not necessarily to be construed aspreferred or advantageous over other embodiments or designs. The terms“at least one” and “one or more” are understood to include any integernumber greater than or equal to one, i.e. one, two, three, four, etc.The terms “a plurality” are understood to include any integer numbergreater than or equal to two, i.e. two, three, four, five, etc. The term“connection” can include an indirect “connection” and a direct“connection.”

References in the specification to “one embodiment,” “an embodiment,”“an example embodiment,” etc., indicate that the embodiment describedcan include a particular feature, structure, or characteristic, butevery embodiment may or may not include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to affect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

For purposes of the description hereinafter, the terms “upper,” “lower,”“right,” “left,” “vertical,” “horizontal,” “top,” “bottom,” andderivatives thereof shall relate to the described structures andmethods, as oriented in the drawing figures. The terms “overlying,”“atop,” “on top,” “positioned on” or “positioned atop” mean that a firstelement, such as a first structure, is present on a second element, suchas a second structure, where intervening elements such as an interfacestructure can be present between the first element and the secondelement. The phrase “direct contact” means that a first element, such asa first structure, and a second element, such as a second structure, areconnected without any intermediary conducting, insulating orsemiconductor layers at the interface of the two elements. It should benoted that the phrase “selective to,” such as, for example, “a firstelement selective to a second element,” means that a first element canbe etched and the second element can act as an etch stop. The terms“about,” “substantially,” “approximately,” and variations thereof, areintended to include the degree of error associated with measurement ofthe particular quantity based upon the equipment available at the timeof filing the application. For example, “about” can include a range of±8% or 5%, or 2% of a given value.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of onemore other features, integers, steps, operations, element components,and/or groups thereof. For example, a composition, a mixture, process,method, article, or apparatus that comprises a list of elements is notnecessarily limited to only those elements but can include otherelements not expressly listed or inherent to such composition, mixture,process, method, article, or apparatus.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form described. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated

While a preferred embodiment has been described, it will be understoodthat those skilled in the art, both now and in the future, can makevarious improvements and enhancements which fall within the scope of theclaims which follow.

What is claimed is:
 1. A magnetic tunnel junction (MTJ) storage element comprising: a reference layer having a fixed magnetization direction; a tunnel barrier layer; and a free layer on an opposite side of the tunnel barrier layer from the reference layer; where the free layer comprises a first region, a second region, and a third region; where the first region comprises a first material configured to include a first predetermined magnetic moment and a first non-fixed magnetization direction; where the second region comprises a second material configured to include a second predetermined magnetic moment and a second non-fixed magnetization direction; where the first predetermined magnetic moment is configured to be lower than the second predetermined magnetic moment; where the third region comprises a third material configured to magnetically couple the first region and the second region; where the first region, the second region, and the third region are configured such that a direction of the first non-fixed magnetization direction changing also initiates the change in the direction of the second non-fixed magnetization direction.
 2. The MTJ storage element of claim 1, where: the first material is further configured to include a first predetermined activation energy; the second material is further configured to include a second predetermined activation energy; and the second predetermined activation energy is configured to be higher than the first predetermined activation energy.
 3. The MTJ storage element of claim 1, where the third region comprises a spacer layer between the first region and the second region.
 4. The MTJ storage element of claim 3, where the spacer layer comprises a nonmagnetic material.
 5. The MTJ storage element of claim 1, where the first material is selected from the group consisting of cobalt (Co), nickel (Ni), iron (Fe) and boron (B).
 6. The MTJ storage element of claim 1, where the first material includes dopants.
 7. The MTJ storage element of claim 6, where a level of the dopants comprises about 10% of the first material.
 8. A magnetic tunnel junction (MTJ) storage element comprising: a reference layer having a fixed magnetization direction; a tunnel barrier layer; and a free layer on an opposite side of the tunnel barrier layer from the reference layer; where the free layer comprises a first region and a second region separated by a third region; where the first region comprises a first material configured to include a first predetermined magnetic moment and a first non-fixed magnetization direction; where the second region comprises a second material configured to include a second predetermined magnetic moment and a second non-fixed magnetization direction; where the first predetermined magnetic moment is configured to be lower than the second predetermined magnetic moment; where the first material is further configured to include a first predetermined activation energy; where the second material is further configured to include a second predetermined activation energy; where the second predetermined activation energy is configured to be higher than the first predetermined activation energy; where the third region comprises a third material configured to magnetically couple the first region and the second region; where the first region, the second region, and the third region are configured such that a direction of the first non-fixed magnetization direction changing also initiates the change in the direction of the second non-fixed magnetization direction.
 9. The MTJ storage element of claim 8, where the third region comprises a spacer layer and the third material comprises a nonmagnetic material.
 10. The MTJ storage element of claim 8, where the first material is selected from the group consisting of cobalt (Co), nickel (Ni), iron (Fe) and boron (B).
 11. The MTJ storage element of claim 8, where the first material includes dopants.
 12. The MTJ storage element of claim 11, where a level of the dopants comprises about 10% of the first material.
 13. A magnetic tunnel junction (MTJ) storage element comprising: a reference layer having a fixed magnetization direction; a tunnel barrier layer; and a free layer on an opposite side of the tunnel barrier layer from the reference layer; where the free layer comprises a first region and a second region separated by a spacer region; where the first region comprises a first material configured to include a first predetermined magnetic moment and a first non-fixed magnetization direction; where the second region comprises a second material configured to include a second predetermined magnetic moment and a second non-fixed magnetization direction; where the first predetermined magnetic moment is configured to be lower than the second predetermined magnetic moment; where the first material is further configured to include a first predetermined activation energy; where the second material is further configured to include a second predetermined activation energy; where the second predetermined activation energy is configured to be higher than the first predetermined activation energy; where the spacer region is configured to provide a predetermined magnetic exchange coupling strength between the first region and the second region; where the first region, the second region, and the third region are configured such that a direction of the first non-fixed magnetization direction changing also initiates the change in the direction of the second non-fixed magnetization.
 14. The MTJ storage element of claim 13, where the spacer region comprises a nonmagnetic material and is configured to magnetically couple the first region and the second region.
 15. The MTJ storage element of claim 13, where the first material is selected from the group consisting of cobalt (Co), nickel (Ni), iron (Fe) and boron (B).
 16. The MTJ storage element of claim 13, where the first material includes dopants.
 17. The MTJ storage element of claim 16, where a level of the dopants comprises about 10% of the first material.
 18. The MTJ storage element of claim 17, where the dopants are selected from the group consisting of aluminum (Al), magnesium (Mg), titanium (Ti), scandium (Sc), calcium (Ca), vanadium (V), chromium (Cr), manganese (Mn), germanium (Ge), silicon (Si), carbon (C), beryllium (Be) and gallium (Ga).
 19. The MTJ storage element of claim 18, where: the first material is selected from the group consisting of cobalt (Co), iron (Fe), nickel (Ni), and boron (B); and the second material is selected from the group consisting of Co, Fe, Ni and B.
 20. The MTJ storage element of claim 18, where the second material comprises: multilayers selected from the group consisting of Co/palladium (Pd), Co/iridium (Ir), Co/platinum (Pt), Co/terbium (Tb), Fe/Tb, Co/gadolinium (Gd), Fe/Gd, and Co/rhodium (Rh); or alloys selected from the group consisting of CoPt, CoPd, FePt, FePd, CoFeTb, and CoFeGd.
 21. A magnetic tunnel junction (MTJ) storage element comprising: a reference layer having a fixed magnetization direction; a tunnel barrier layer; and a free layer on an opposite side of the tunnel barrier layer from the reference layer; where the free layer comprises a first region, a second region, and a third region; where the first region comprises a first material configured to include a first predetermined magnetic moment; where the second region comprises a second material configured to include a second predetermined magnetic moment; where the first predetermined magnetic moment is configured to be lower than the second predetermined magnetic moment; where the third region comprises a third material configured to magnetically couple the first region and the second region; where the reference layer is configured to, based on the MTJ storage element receiving a write pulse of current having a predetermined duration and a predetermined magnitude, generate enough spin torque electrons to tunnel through the tunnel barrier layer into the first region to begin switching a first non-fixed magnetization direction of the first material of the first region; where the first region, the second region, and the third region are configured such that the direction of the first non-fixed magnetization direction changing initiates a change in a direction of a second non-fixed magnetization direction of the second region.
 22. The MTJ storage element of claim 21, where a non-switching state of the MTJ storage element comprises the first non-fixed magnetization direction pointing in a same direction as the second non-fixed magnetization direction. 